Bimetallic complexes containing a Lewis acid in combination with metal carbonyl are highly active catalysts for the ring-expanding carbonylation of strained heterocycles, including epoxides, aziridines, oxetanes and lactones. In particular, such bimetallic catalysts comprising a cationic aluminum complex as a Lewis-acidic component and a carbonyl cobaltate anion are useful for the mono- and bis-carbonylation of epoxides to form beta-lactones and succinic anhydrides respectively (Rowley et al., J. Am. Chem. Soc., 2007, 129, 4948-4960).
Continuous production of these beta-lactones and succinic anhydrides is possible by the continuous addition of epoxide and carbon monoxide feedstocks, and the continuous removal of the beta-lactone or succinic anhydride products. However, the carbonylation catalysts used for these reactions have a limited effective lifespan, and accordingly, the continuous production of carbonylation products slows down over time or ceases entirely once the active catalyst species is no longer present to catalyze the reaction. These active catalysts are expensive, so the addition of a whole new amount of active catalyst is not economically preferable.
Furthermore, many of the active catalysts are air sensitive and it is undesirable to have to produce and handle them in a separate step prior to adding them to the continuous carbonylation reactor. Such a separate step can be problematic at commercial production volumes. As such, there remains a need for methods of maintaining effective amounts of carbonylation catalysts in a continuous carbonylation reactor that are practical and efficient for large-scale use.